Transmitter and Oscillator Systems A transmitter is an important subsystem in a wireless system. In any active wireless system, a signal will be generated and transmitted through an antenna. The signal’s generating system is called a transmitter. The specifications for a transmitter depend on the applications. For long-distance transmission, high power and low noise are important. For space or battery operating systems, high efficiency is essential. For communication systems, low noise and good stability are required | RF and Microwave Wireless Systems. Kai Chang Copyright 2000 John Wiley Sons Inc. ISBNs 0-471-35199-7 Hardback 0-471-22432-4 Electronic CHAPTER SIX Transmitter and Oscillator Systems TRANSMITTER PARAMETERS A transmitter is an important subsystem in a wireless system. In any active wireless system a signal will be generated and transmitted through an antenna. The signal s generating system is called a transmitter. The specifications for a transmitter depend on the applications. For long-distance transmission high power and low noise are important. For space or battery operating systems high efficiency is essential. For communication systems low noise and good stability are required. A transmitter can be combined with a receiver to form a transceiver. In this case a duplexer is used to separate the transmitting and receiving signals. The duplexer could be a switch a circulator or a diplexer as described in Chapter 4. A transmitter generally consists of an oscillator a modulator an upconverter filters and power amplifiers. A simple transmitter could have only an oscillator and a complicated one would include a phase-locked oscillator or synthesizer and the above components. Figure shows a typical transmitter block diagram. The information will modulate the oscillator through AM FM phase modulation PM or digital modulation. The output signal could be upconverted to a higher frequency. The power amplifiers are used to increase the output power before it is transmitted by an antenna. To have a low phase noise the oscillator or local oscillator can be phase locked to a low-frequency crystal oscillator. The oscillator could also be replaced by a frequency synthesizer that derives its frequencies from an accurate high-stability crystal oscillator source. The following transmitter characteristics are of interest 1. Power output and operating frequency the output RF power level generated by a transmitter at a certain frequency or frequency range. 172 TRANSMITTER .
